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I think its ambiguous, Kerim, but seeing has Shapechange has language describing changing forms during the duration of the spell, and Alter Self does not, I'd assume no. That's just an opinion, though.

Does the spell Alter Self allow you to change forms while the spell is active? If, say, you started as a human, then altered to a drow, could you alter yourself to a high elf?

No.
A spell does what its description says it does.

Or, look at it this way:
Shapechange refers to Polymorph which, in turn, refers to Alter Self. Since Shapechange explicitely states that the assumed form can be changed (and neither Polymorph nor Alter Self states anything in this direction), the 'lesser versions' only allow switching forms once: When casting the spell.

Touch spells trigger AoO whenever another spell would normally do so, meaning most of the time unless actions are taken to avoid it, in which case all the normal methods apply, starting with simple concentration checks.

If you're putting meaningful resources into spellcasting, those concentration checks are already something you should be investing in, and they're not all too terribly difficult.
If you do end up having serious difficulty with them, there are some feats that could help you, granting bonuses, allowing you to take 10, or even granting the benefit of success (denying the AoO) without rolling under particular circumstances (the one I'm thinking of requires that you wield a shield)